Browning leaf tips in a containerized plant is almost always a sign of water stress (too much or too little), but if you are growing your tree indoors, it could also indicate that the air is too dry. I've grown lots of avocado trees from pits and used them indoors as foliage plants. When the air is too dry, the leaf tips turn a crispy brown. You can help increase the humidity by placing the pot on a pebble-filled tray in which you keep a half-inch of water. As the water evaporates it increased humidity around the plant. If your avocado tree is outdoors rather than in, the problem can be solved by keeping the soil consistently moist, but not soggy wet. Hope this helps!